1. Field of the Invention
The present invention relates to a method of measuring and checking a shape of a fine object or an article by using an interference image between light having information about the shape of the object and reference light. The invention also relates to an apparatus for carrying out such a method.
2. Related Art Statement
Various types of apparatuses for measuring and checking a shape of objects utilizing an interferometer have been proposed. For instance, some known shape measuring apparatuses have been disclosed in D. Malacara, "Optical Shop Testing", John Wiley and Sons, New York (1978). Particularly, a phase measurement using the fringe scan has been widely used in measurements of a fine structure of an object, because depressions and protrusions of an object surface can be measured with a precision which is higher than a hundredth of a wavelength.
C. Bouwhius et al have reported in "Principles of Optical Disc Systems", Intern. Trens in Optic, Acad. Press (1991) that when an object surface has a step, diffracted light contains a phase jump (singular point) near the step. This singular point of a very small size is often generated not only at the step, but also in a close vicinity of a point at which optical property of an object under inspection is discontinuous. For instance, at a boundary of two different optical materials, a singular point occurs. Therefore, by measuring the position of a singular point with a high precision, an optical discontinuous point can be measured precisely.
V. P. Tychinsky has reported in "Computerized Phase Microscope for Investigation of Submicron Structure", Optics Communications, Vol. 74, (1989), pp. 37-40 that a phase measurement can observe a fine structure of a size smaller than the Rayleigh limit, which had been considered as the resolution limit. Recently, the size of IC patterns and thin film magnetic head gaps has been reduced more and more, so that it is difficult to observe these articles by conventional optical microscopes. The phase measuring technique has a possibility for providing a solution for such a problem.
FIG. 1 is a schematic view showing a known shape measuring device. This shape measuring device utilizes the Twyman-Green interferometer. A parallel coherent light beam emitted by a laser 1 is expanded by a beam expander 2. The expanded parallel laser beam is made incident upon an interference optical system 3 formed by a half mirror and is divided thereby into an inspection laser beam which is directed toward an object 4 under inspection along an inspection optical path 5 and a reference laser beam which is directed toward a reference body 6 along a reference optical path 7.
The inspection and reference laser beams are reflected by the object 4 and reference body 6, respectively and are made incident again upon the interference optical system 3 along the inspection and reference optical paths 5 and 7, respectively. At the interference optical system 3, these laser beams are composed with each other to produce a composite laser beam due to an interference. The composite laser beam is then made incident upon an objective lens 8 and an interference image of the object 4 and the reference body 6 is formed on an image sensing device 9. An image signal obtained by the image sensing device 9 is supplied to an image display device 11 via a controller 10 and the interference image is displayed thereon.
In the interference image displayed on the image display device 11, there are produced interference fringes in accordance with a local difference in an optical path length between the inspection optical path 5 and the reference optical path 7. Therefore, during a time when the reference body 6 is moved in the direction of the optical axis by driving a phase modulator 12 from the controller 10 to vary the difference in optical path length finely, a plurality of interference images are picked-up by the image sensing device 9. This operation is generally called the fringe scan. Then, a phase distribution in a vicinity of a surface of the object 4 can be calculated from the interference images. Methods of calculating the phase distribution from plural interference images obtained by using the fringe scan have been described in detail in Katherine Creath, ""PHASE-MEASUREMENT INTERFEROMETRY TECHNIQUES", Progress in Optics XXVI, Amsterdam 1988, pp. 351-393 and JP-A 5-232384. A shape of an object under inspection may be estimated by deriving one or more phase singular points in the phase distribution.
However, the known shape measuring apparatus using a phase singular point has the following problem. That is to say, in the known apparatus, only the lateral position of a phase singular point is detected and the position of an optical discontinuity on the surface of an object under inspection is estimated from the thus detected lateral position of the phase singular point. Therefore, if a recess-like microstructure is formed on an object surface, it is possible to measure the width of the recess, but the depth of the recess could not be measured.
As stated above, without knowing a correlation between the phase distribution of the electric field in a vicinity of the surface of an object under inspection and the shape of the surface of the object correctly, it is impossible to measure the shape of the object in an accurate manner. However, the known shape measuring apparatuses using an interferometer show a detected phase distribution as it is, which does not always give the correct shape of the object under inspection. For instance, in a detection of the lateral position of an optical discontinuity using a phase singular point, the detected lateral position of the phase singular point does not always coincide with the actual point of the optical discontinuity on the object surface, and thus a measured width of a recess formed on the object surface can be different from the actual value. Moreover, by the known shape measuring apparatuses, the depth of a recess formed on an object surface could not be measured at all.